Surface modification is used to obtain desired surface properties, such as non-fouling properties. Modification of surfaces by plasma energy, including plasma treatment and plasma polymerization, has been described in prior arts. For example, in U.S. Pat. No. 6,482,531, plasma polymerization methods are described in which a pulsed plasma discharge is used to polymerize a monomer. In U.S. Pat. No. 5,002,794 and U.S. Pat. No. 5,153,072, plasma polymerization methods involving controlling the temperature of the substrate and the reactor so as to create a temperature differentially between the substrate and reactor are disclosed.
In the prior arts of plasma polymerization, the monomer is introduced to the plasma chamber through an inlet with valve(s) and/or flow regulator(s). The valve or flow regulator is first closed, and the plasma chamber is pumped down to sufficiently high vacuum with the monomer being separated by the valve/flow regulator from the chamber. Then the valve or flow regulator is opened to allow the vapor of the monomer to enter the vacuum chamber. After the pressure of the monomer in the chamber stabilizes, the plasma power is turned on to generate plasma in the chamber. The plasma in the chamber induces the deposition and polymerization of the monomer on surfaces exposed in the plasma chamber.
One limitation of the prior arts is that it does not work well if the vapor pressure of the monomer at room temperature is relatively low. Therefore, monomers used in plasma polymerization have been limited to compounds with relatively high vapor pressure, such as allyl alcohol, allylamine, acrylic acid and octadiene.
One way to overcome this limitation is to use elevated temperature to improve the flow of low-vapor-pressure monomers to the chamber. However, it requires extra equipment to maintain the whole plasma chamber at an elevated temperature, and the partial pressure of the monomer in the chamber is still limited by the conductance of the monomer inlet system, including the pipes and valves and/or flow regulators.
Reducing vacuum pumping rate may increase the partial pressure of the monomer, but it also increases the impurity (especially oxygen) in the plasma. Due to insufficient monomer vapor pressure and/or high impurity of the chemical vapor in the chamber, the efficiency of surface coating is low, resulting in low density of the desired functional groups coated on the surface.
In the case of producing a non-fouling surface by covalently attaching ethylene glycol groups, insufficient coating density results in ineffective functions due to pin holes that provide binding sites. The surfaces produced this way may not be able to reduce the binding of macromolecules or micro-organisms compared to the untreated surface; or it may reduce the binding but not to the desired level.